1. Field of the Invention
The present invention relates generally to a sensor system for monitoring the structural integrity of an underground waste containment barrier, and leakage therefrom of waste products or byproducts, and for improved characterization of zones of interest.
2. Background Art
It is often necessary to form a containment barrier around a hazardous waste site to stop or prevent the migration of contaminants into the nearby soil and water tables. The containment barrier must prevent the migration of contaminants both horizontally and vertically away from the waste site. Therefore, a properly constructed containment barrier may be compared to a huge bathtub, with the hazardous waste contained within four side walls and a generally horizontal floor.
A typical, currently-used method of containment is to physically remove the hazardous waste and haul it to a permitted storage facility. However, such method is costly, impractical, and dangerous. Digging up sites with buried drums, radioactive dusts, or other airborne wastes may actually release the contaminants, spreading them into the atmosphere and through the soil.
In response to this problem, a number of suggestions have been made for placing containment barriers around hazardous waste sites, without removing the waste. One approach for doing this is disclosed in International Publication Nos. WO 94/19547 and WO 93/00483 by Halliburton Nus Environmental Corp. The Halliburton system uses a row of high pressure jets to shoot a slurry into soil surrounding a hazardous waste site, somewhat liquefying the surrounding soil. The slurry cuts a path through the soil as it intermixes with the liquified soil. Gravity and/or mechanical means pull the row of high pressure jets through the mix of liquefied soil and slurry, after which the liquefied soil and slurry harden into a protective barrier.
The above-described system has a number of shortcomings, including the possibility of further spreading contaminants by the use of hydraulic jets, the difficulty of maintaining balance between the amount of slurry needed for cutting and the amount of slurry needed for hardening the soil, the difficulty of providing a barrier of consistent strength since it would depend in part upon the soil composition encountered and the amount of slurry deposited, and, finally, the lack of testing of excavated soil to know whether soil surrounding the waste site has become contaminated.
Another suggested approach for installing a containment barrier around a hazardous waste site is disclosed in co-pending patent application Ser. No. 08/925,101, filed Sep. 8, 1997. In this approach, a multi-layer containment barrier is put in place under a hazardous waste site without disturbing any buried waste, in a simple and efficient fashion. The disclosure in the above-noted co-pending patent application is incorporated herein by reference.
In any approach to holding hazardous waste, it would be desirable to monitor the site in terms of both the structural integrity of any containment barrier put in place about the waste material, and leakage of contaminants away from the site. Additionally, it would be desirable to monitor material being excavated from around a waste site in preparation for emplacement of a containment barrier for the site, to determine the extent of contamination of surrounding soils and thus the possible need to extend the containment barrier to a location completely surrounding all contaminated materials and soils. Finally, it would be desirable to efficiently and inexpensively install a long-term monitoring system soon after or simultaneously with the installation of the containment barrier.
It is an object of the invention to provide a sensor system for sensing a variety of physical parameters of a buried waste containment site.
It is also an object of the invention to provide such a sensor system especially suitable for use in connection with a containment barrier disposed under and around a buried waste site.
It is a further object of the invention to provide such a sensor system for monitoring the structural integrity of such a containment barrier.
It is also an object of the invention to provide such a sensor system for sensing leakage of contaminants from a buried waste containment site.
It is still another object of the invention to provide such a sensor system, in accordance with, one aspect thereof, for monitoring soil and material excavated from a buried waste containment site.
It is an additional object of the invention to provide such a sensor system, in accordance with another aspect thereof, for sensing physical parameters of soil being excavated, during the excavation process.
It is, a further object of the invention to provide such a sensor system which may be readily installed at a buried waste containment site simultaneously with the installation of a containment barrier.
It is also an object of the invention to provide such a sensor system in which sensors may be installed and removed after the buried waste containment site is in place.
The above and other objects of the invention are realized in a specific illustrative embodiment of a sensor system for a buried waste containment site having a bottom wall barrier and/or sidewall barriers for containing hazardous waste. The sensor system includes one or more sensor devices disposed in one or more of the barriers for detecting a physical parameter either of the barrier itself or of the physical condition of the surrounding soils and buried waste, and for producing a signal representing the physical parameter detected. Also included is a signal processing device for receiving signals produced by the sensor device and for developing information identifying the physical parameter detected, either for sounding an alarm, displaying a graphic
representation of the physical parameter detected on a viewing screen and/or a hard copy printout, etc.
In accordance with one aspect of the invention, the sensor device disposed in one or more of the barriers comprises a strain or crack transducer for detecting strain or cracking and thus possible leakage locations in the barrier in which the transducer is disposed. One embodiment of such a transducer includes a grid of detecting elements disposed in the barriers to detect strains wherever they might occur.
In accordance with another aspect of the invention, one or more access tubes are disposed in or below the barriers with at least one end of the tubes extending from the barriers to allow access thereinto. Sensor devices are then disposed in the access tube or tubes and coupled to the signal processing device through the one end of the tubes. The access tubes provide protection for the sensor device without inhibiting operation thereof. Also, use of access tubes allows for selective removal and deployment of a variety of sensors.
In accordance with still another aspect of the invention, the sensor device is adapted to detect radiation that may be leaking or may have already leaked through the barriers, and/or the presence of RCRA metals. Also, a sensor device may be provided to detect volatile organic compounds using fiber optic spectroscopy deployed in the access tubes.
In another embodiment of the invention, conveyor apparatus is provided for removing and carrying away excavated earthen material. Disposed above the conveyor apparatus and above any material being, carried, by the conveyor apparatus is one or more sensor devices for detecting various conditions and components of the material being carried. The sensor device is coupled to a processing device for developing information identifying the condition or components detected by the sensor device, just as with the sensor device disposed in the containment barriers described above.
In another aspect of the invention, sensor detectable tracers could be used to verify barrier integrity. Specifically, tracers could be placed within the barrier with sensors outside the barrier to determine whether the tracers have migrated through a breach in the barrier, or stayed in place.
In a further aspect of the invention, sensors or sensor arrays are installed in or about a barrier simultaneously with the installation of the barrier. For example the sensors or sensor arrays could be disposed between layers of a multi-layer barrier as the barrier is being installed in a trench dug for that purpose.
As indicated earlier, one approach to installing a containment barrier around a waste site involves the use of high pressure jets shooting a slurry into soil surrounding the waste site. This is also known as grouting, and typically involves a grouting beam or arm which carries the jets and which is moved along a locus to both remove soil and produce the containment barrier with a mixture of slurry and soil. In accordance with an aspect of the present invention, a sensor or sensors are disposed on the grouting arm to detect physical properties of the soil through which the arm moves, to thus determine whether contaminants have leaked from the waste site into the surrounding soil.